Configure link check to use Accept-Encoding header for docs.github.…

…com links

The link check job of the "Check Markdown" workflow has had frequent intermittent spurious failures recently, caused by
links under the docs.github.com domain returning 403 HTTP status.

Others experiencing the same problem reported that they were able to work around the problem by providing a custom
`Accept-Encoding` HTTP request header. Although I was not able to find any explanation of why, it does appear to resolve
the problem.

Since the problem seems to be permanent and the only other workarounds I have identified are unappealing (considering
links returning 403 statuses alive, or ignoring all docs.github.com links), I think it is worth a try.

.markdown-link-check.json

@@ -1,4 +1,12 @@
 {
+  "httpHeaders": [
+    {
+      "urls": ["https://docs.github.com/"],
+      "headers": {
+        "Accept-Encoding": "gzip, deflate, br"
+      }
+    }
+  ],
   "retryOn429": true,
   "retryCount": 3,
   "aliveStatusCodes": [200, 206]